In common practice, wastewater treatment plants (WWTPs) include a biological process step in which part of the wastewater containing solid matter, suspended and soluble organics and nutrients, is treated by activated sludge (consisting of mainly micro-organisms). This process can take place either anaerobically or aerobically. The most widely applied process for the aerobic treatment of wastewater is called the ‘conventional activated sludge’ (CAS) process. It involves air or oxygen being introduced into a biological treatment reactor which contains a mixture of screened and sometimes primary treated sewage or industrial wastewater and purifying biomass, also referred to as ‘activated sludge’. The mixed liquor suspended solids (MLSS) develop into a biomass-containing floc, which typically grows in suspended fluffy aggregates. The subsequent settling tank (usually referred to as “final clarifier”) is used to allow the biological flocs to settle, thus separating the purifying sludge from the treated water. The settled sludge is recycled towards the biological process as ‘return activated sludge’ (RAS). To keep the biomass in the treatment reactor at a desired level during biomass growth, periodically part of the RAS is wasted as ‘waste activated sludge’ (WAS).
The CAS process is applied in a variety of configurations, comprising one or multiple tanks in parallel or sequential treatment train(s). Such tanks can for example be operated as plug-flow reactor, as continuous stirred tank reactor (CSTR) or as sequencing batch reactor (SBR). Although the CAS process is widely used, it has several important drawbacks, like: poor settling sludge characteristics, limitation to low MLSS concentrations, the tendency to develop floating sludge and a defined activated sludge residence time. These drawbacks are briefly described hereafter.
Poor Settling Sludge Characteristics
Due to its floc-like structure, the settling characteristics of activated sludge are relatively poor, even when the plant is operating well. This results in the need for large final clarifiers and accordingly high construction costs and large plant footprint. Many improvements in the past therefore focused on achieving improved separation techniques. One of them is the use of microfiltration to separate the activated sludge from the treated water in a Membrane Bio Reactor (MBR). Another one is the addition of chemicals to improve the biomass settling characteristics. In WO96/14912 a method is described that improves the settling properties of activated sludge by extracting gas and creating higher biomass density. The method of selectively withdrawing poorly settling sludge is described in EP1627854.
Limitation to Low MLSS Concentrations
The CAS process is limited to a relatively low concentration of MLSS, typically 3-5 g MLSS/L. Higher concentrations of MLSS lead to an unfavourable prolonged sludge holdup in final clarifiers and, especially during conditions with higher than average hydraulic flows, to potential sludge washout. State of the art measures to increase the level of MLSS focus on the application of microfiltration for sludge/water separation (Membrane Bio Reactors) instead of settling or the use of submerged carrier material to enhance the biomass concentration, as for example described in WO03/068694.
Floating Sludge
The CAS process coincides with a periodical occurrence of floating or very difficult to settle ‘bulking sludge’, a phenomenon caused by an increased growth of filamentous micro-organisms in the activated sludge flocs. Typical counteracting measures include chemical oxidation to destroy mainly the filamentous organisms or use of special biomass selection reactors prior to the activated sludge in which the growth of filamentous micro-organisms is reduced.
Defined Activated Sludge Residence Time
The CAS process for nutrient removal is typically designed with a defined activated sludge residence time in the system of 5-15 days. This time period sets a limit to the accumulation of favourable species of micro-organisms with low growth rates, which cannot be maintained in the treatment system. Measures to extend the sludge residence time include the Membrane Bio Reactor, the addition of submerged carrier material for attached growth and the use of bio-augmentation. In these bio-augmentation processes, a specific micro-organism population is cultivated and often immobilized in bio-augmentation reactors. The reactors are fed with specific substrate or integrated waste side streams from the wastewater treatment facility and then dosed to the CAS system, as described in e.g. EP0562466. Another example of such an in-situ bio-augmentation process is described in WO00/05177: it describes an external bio-augmentation reactor to enrich specific organisms in the activated sludge matrix.
The drawbacks of typical CAS systems are overcome to a large extent by the aerobic granular biomass (AGB) process and system as developed by Delft University of Technology (WO2004/024638). In this process granular biomass with a typical size of 0.2-3.0 mm is grown that has very different characteristics from the flocs as grown in CAS. For example the settling velocity of the applied granules is in the range of 5.0-50.0 m/h (in comparison: typical for CAS would be 0.5-1.0 m/h). Sludge volume indices (SW) for aerobic granular biomass are 70 ml/g or lower and typically are comparable in value after 5 and 30 minutes of settling time. In addition, MLSS concentrations can be kept at levels 2-4 times higher than in CAS systems, resulting in approx. 2-4 times more ‘purification power’. Furthermore, both the layered structure of granules in aerobic, anoxic and anaerobic zones and the range in granule sizes result in a large distribution of sludge ages. This enables specific and favourable micro-organisms with low growth rates to survive in the AGB process.
However, one drawback of the AGB process is the fact that the granules need to be grown in a discontinuously fed system, in sequencing batch reactors. It has been reported that AGB can only develop and be maintained in batch-wise operations, during which slow growing micro-organisms are selected at high feed concentrations followed by a famine regime during non-feed conditions (see: WO2004/024638). Such conditions can by definition not be established easily in continuously fed CAS systems.
Therefore, the technology cannot easily be used to retrofit continuously fed CAS systems into systems aimed at growing AGB. Replacement of the widely used continuous CAS systems would mean large capital disinvestment. Efforts to develop a continuously fed AGB system have been reported in literature but so far prove not feasible at practical conditions. Reference is made to a study on the formation and stability of aerobic granules in a continuous system: (N. Morales, et al., Separation and Purification Technology, volume 89, page 199-205, 2012). Efforts also have been made to replace the activated sludge in continuous MBR systems with aerobic granular biomass in order to reduce membrane fouling. It was investigated whether the activated sludge in the continuous MBR systems could be replaced by granular biomass grown in cultivation reactors or grown in granular biomass reactors. The results showed that it was not feasible to keep the aerobic granules in the MBR system: the granules deteriorated quickly (Reference: Xiufen et al., Characteristics of Aerobic Biogranules from Membrane Bioreactor System, Journal of Membrane Science, 287, page 294-299, 2006). As a consequence, in the current state-of-the-art, upgrading performance of existing CAS systems using aerobic granular biomass is only possible by retrofitting CAS systems into sequencing batch operated AGB reactors.
Even if in a hypothetic case granular biomass would be able to survive in CAS, the size and settling characteristics of the granules are such that in many CAS the mixing intensity is not sufficient and they will settle to the bottom and as such becoming inactive for the treatment process.
An assumed drawback of batch operated systems like the AGB system is the sensitivity to off-spec high hydraulic load fluctuations. This is because all operations take place in one tank and the feed to one tank is discontinuous. This is different from CAS systems equipped with large final clarifiers, which clarifiers can act as buffer tank to prevent sludge loss. This drawback can be counteracted by installing feed buffer tanks or adjusting feed patterns over the multiple AGB process tanks.
JP-A 2009-090161 discloses an aerobic wastewater treatment comprising a series (not a parallel arrangement) of aeration tanks. Granular flaked sludge is produced in an oscillating bed with carrier material in the first aerated tank and fed to the second tank. JP-A 2007-136368 discloses an aerobic wastewater treatment wherein sludge is granulated in a contact tank, and sludge is then fed to a downstream reactor; surplus granular sludge from the aerobic reactor is returned to the contact tank. WO 2007/029509 discloses an aerobic wastewater treatment process with sludge return, using a partitioned aerated tank and microorganisms immobilised on a carrier in the first compartment.